A programmable controller or PLC (“Programmable Logical Controller”) is an automatic control facility capable of driving, controlling and/or monitoring one or more processes to be controlled.
Of generally modular design, a PLC programmable controller is composed of various modules which intercommunicate through a transmission bus, called a “backplane” bus in this field. The modules are fixed mechanically in a rack, which comprises a printed circuit which also supports the backplane bus as well as the connection elements intended to cooperate with connectors generally present on the rear part of the modules so as to effect the necessary link between the modules and the bus. The number of modules depends of course on the size and the type of process to be automated.
Typically, a programmable controller can comprise:                A power supply module providing the various voltages to the other modules through the backplane bus.        A central unit module UC which comprises embedded software (“firmware”) integrating a real-time operating system OS, and an application program, or user program, containing the instructions to be executed by the embedded software to perform the desired control operations. The UC module also generally comprises a connection on the front face to programming tools of personal computer PC type.        Input/output I/O modules of various types as a function of the process to be controlled, such as digital I/Os or analogue TORs for counting, etc. These I/O modules are linked to sensors and actuators participating in the automated management of the process.        One or more modules for communicating with communication networks (Ethernet, CAN, etc.) or man-machine interfaces (screen, keyboard, etc.).        
By way of example, an input/output module can comprise between 1 and 32 I/O pathways, a PLC controller that may be capable, depending on the model, of managing several hundred I/O pathways. If required, several racks are therefore connected together in one and the same PLC. Thus, as a function of the application and the process to be automated, a PLC controller can comprise a large number of modules.
Currently, analogue input modules comprise several pathways isolated from one another with the aid of optical isolation static relays also called “OptoMos” (trademark). Each pathway of the module is controlled by two static relays, one of the poles of whose MOSs is common. These components have an almost unlimited lifetime but each exhibit a not inconsiderable leakage current. Thus, when the module comprises more than four pathways, the unit leakage currents of each of the static relays add together thus giving rise to consequent errors in the measurements.